As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Typically, the manufacturing process for modern semiconductor devices includes the formation of a number of layers or “thin films” on a silicon wafer. The thin films can include oxide, nitride, and/or metal layers, among others. To ensure proper performance of the finished semiconductor devices, the thickness and composition of each thin film formed during the manufacturing process must be tightly controlled.
Modern thin films have reached the point where the accuracy and reproducibility of thin film metrology (i.e., measurement and/or inspection) can be limited by contamination on the surface of the thin film. For example, airborne molecular contamination (AMC) such as water and other vapors can be absorbed onto the thin film, creating a contaminant layer that adversely affects thin film analysis techniques such as optical ellipsometry, optical reflectometry, grazing-incidence x-ray reflectometry (GXR), x-ray fluorescence (XRF), electron microprobe analysis (EMP), and non-contact electrical analysis—all of which operate by directing a probe beam (optical, x-ray, electron or corona discharge) at the surface of the thin film to be measured. The contaminant layer can also interfere with measurements techniques that physically contact the surface of the thin film, such as contact-based electrical analysis (e.g., spreading resistance analysis).
Conventional methods for cleaning thin films include heating the entire wafer in an oven to a temperature of about 300° C. to vaporize any contaminant layer. FIG. 1a shows a conventional oven-based wafer cleaning system 100a used to prepare a wafer 110 for thin film analysis, as described in U.S. Pat. No. 6,325,078, issued Dec. 4, 2001 to Kamieniecki. Wafer 110 includes a thin film layer 112 formed on a silicon substrate 111, and a contaminant layer 113 formed on the surface of thin film layer 112. Wafer cleaning system 100a comprises a wafer stage 120, and multiple heat lamps 130. Wafer stage 120 positions wafer 110 under heat lamps 130, where thermal radiation from heat lamps 130 heats wafer 110 to vaporize contaminant layer 113. It is believed that this cleaning process is aided by optical photons from heat lamps 130 breaking bonds between contaminant layer 113 and thin film layer 112.
FIG. 1b shows another conventional wafer cleaning system 100b used to prepare wafer 110 for thin film analysis, as described in U.S. Pat. No. 6,261,853, issued Jul. 17, 2001 to Howell et al. Just as described with respect to FIG. 1a, wafer 110 includes a thin film layer 112 formed on a silicon substrate 111 and a contaminant layer 113 formed on the surface of thin film layer 112. Cleaning system 100b incorporates a stage 140 that includes a heating element 141. Heat generated by heating element 141 is conducted through stage 140 into wafer 110, thereby providing the heating required to vaporize contaminant layer 113. A heat exchanger can be coupled to stage 140 to capture excess heat from heating element 141 to minimize undesirable heating of cleaning system 100b itself and the surrounding environment.
Although wafer cleaning systems 100a and 100b use different thermal energy sources (i.e., heat lamps 130 and heating element 141, respectively), both systems perform a bulk heating operation to remove contaminant layer 113. The large thermal control components (e.g., lamps, heated stages, heat exchangers, etc.) typically used for bulk wafer heating undesirably increase the cleanroom space required for these conventional cleaning systems. Further exacerbating the problem of excess equipment size, conventional cleaning systems are sometimes stand-alone units used in conjunction with a thin film analysis tool. Therefore, conventional cleaning systems can significantly increase the total footprint required for a complete thin film analysis system. The use of a separate cleaning system also has an adverse effect on throughput, as time must be spent transferring the wafer to and from the cleaning system. In addition, contaminants can redeposit on the cleaned wafer while it is being transferred from the cleaning-system to the film analysis tool.
In an attempt to somewhat alleviate these equipment size and recontamination problems, attempts have been made to combine wafer cleaning and measurement capabilities in a single tool. For example, the aforementioned U.S. Pat. No. 6,261,853 describes integrating cleaning system 100b with an existing metrology tool (Opti-Probe 5240 from Therma-Wave, Inc.). Also, the Quantox XP tool from KLA-Tencor integrates a wafer cleaning system similar to cleaning system 100b with a non-contact electrical film measurement system. However, any bulk wafer heating system must still incorporate the aforementioned (large) thermal control components. Furthermore, even if a combined system is used, the bulk heating operation can significantly degrade overall wafer processing throughput. Several seconds are required to heat the wafer to the temperature required for removal of the contaminant layer, and another several seconds are required to cool down the wafer after cleaning. Any wafer handling operations that must be performed during and after the cleaning operation (e.g., transferring the wafer from the cleaning system to the thin film analysis system) further reduces the throughput. Note also that any delays after cleaning allow contaminant regrowth on the wafer.
To improve throughput and reduce system footprint, a laser cleaning system can be incorporated into a metrology system. FIG. 2a shows an integrated laser cleaning metrology system 200, which is described in detail in co-owned and co-pending U.S. patent application Ser. No. 10/056,271. Metrology system 200 comprises a stage 220, an energy beam source 230, and an analysis module 240. The compact components used in an energy-beam based cleaning system (such as energy beam source 230) can be efficiently integrated into metrology system 200 to minimize system footprint.
Stage 220 holds a test sample 210 that comprises a thin film layer 212 formed on a substrate 211 and a contaminant layer 213 formed on the surface of thin film layer 212. Energy beam source 230 directs an energy beam 231 at a spot 214a on contaminant layer 213 to expose the underlying portion of thin film layer 212. Then in FIG. 2b, stage 220 positions test sample 210 under analysis module 240 so that a measurement beam 246 can be directed onto thin film layer 212 through an opening 214b formed by the laser heating of spot 214a during the preceding cleaning operation (as shown in FIG. 2a). Since only a localized portion of contaminant layer 213 is cleaned, the long heating and cooling times associated with conventional cleaning systems can be avoided to improve throughput, and the only delay between cleaning and measurement is the time required to reposition test sample 210 under analysis subsystem 240—typically 1–2 seconds.
However, as metrology parameters become ever more sensitive to AMC, even this 1–2 second delay between cleaning and measurement can allow an excessive amount of AMC recontamination onto the thin film layer. For example, many modern metrology operations require test sample surface stabilities on the order of a tenth of an angstrom. However, AMC regrowth rates can be in the 1 Å/sec range, in which case a repositioning delay of even a second can lead to significant measurement inaccuracies. Furthermore, since the measurement process itself can take a few seconds to complete, significant AMC regrowth can actually take place over the course of the measurement operation.
Accordingly, it is desirable to provide a method and system for performing thin film metrology that avoids the aforementioned problems associated with AMC contamination and regrowth.